Method for making a precision casting



Sept 1968 R. E. CHIRISTIAN 3,402,755

METHOD FOR MAKING A PRECISION CASTING Filed Jan. 13, 1966 2 Sheets-Sheet l NVENTOR.

FTTORNE Y5.

P 24, 1968 R. E. CHRISTIAN 3,402,755

METHOD FOR MAKING A PRECISION CASTING Filed Jan. 13, 1966 2 Sheets-Sheet 2 IN VEN TOR.

P067726; I 6272;52 :071 BY a j fi United States Patent 3,402,755 METHOD FOR MAKING A PRECISION CASTING Rodney E. Christian, Birmingham, Mich., assignor to Traub Manufacturing "Company, a corporation of Michigan Filed Jan. 13, 1966, Ser. No. 520,462 1 Claim. (Cl. 164-35) ABSTRACT OF THE DISCLOSURE A method of making a precision casting wherein a nonmetallic destructible pattern is coated with a thin layer of metal by a rnetallizing technique. The pattern is then destructibly removed leaving a thin hollow shell mold that is filled with a molten metal. After the poured metal solidifies the mold is also destructibly removed.

This invention relates to a method for casting articles and more particlularly to an improved method for forming precision castings having close tolerance surface finishes and shapes.

In many instances a finished article is formed by a casting process and in some of these cases the surface finish and shape of the finished cast article is relatively unimportant. In many instances, however, it is desirable to use a casting process but a good surface finish is required so as to minimize the subsequent machining or finishing operations.

The casting of finger rings is a good example of an instance where the article is cast, but it is desirable to minimize the subsequent finishing operations. Such rings have commonly been cast by a process known as the lost wax process. With the lost Wax process, a wax pattern is formed in the shape of a ring. The wax is then surrounded by an investment of plaster of paris or the like, and the wax is subsequently removed from the plaster of paris by melting it. The gold or other precious metal which forms the ring is then poured into the plaster of paris mold filling the cavity left by the lost wax. Although the wax pattern has an extremely smooth surface finish which conforms to the master from which it is made, the plaster of paris surrounding the wax has a rough texture which the cast precious metal assumes. Therefore, it has been necessary to perform considerable polishing operations upon the cast ring so that it will be salable. These subequent finishing operations add greatly to the cost of producing the ring and require expert personnel who are not always available.

It is, therefore, the principal object of this invention to provide an improved casting process in which the surface finish of the cast article is of high quality.

It is a further object of the invention to provide an improved low cost precision casting method.

It is a still further object to provide an improved precision casting process using the lost wax method.

The method of precision casting embodying this invention comprises the steps of forming a pattern and depositing a metal upon the pattern. The pattern is then removed from within the metal to form a hollow metallic mold into which a molten material is poured. In a preferred embodiment of the invention the metallic mold is removed subsequent to the pouring by immersing it in a caustic that will dissolve it but not affect the poured material.

Other objects and advantages of this invention will become more apparent as this description proceeds, particularly when considered in conjunction with the accompanying drawings, wherein:

FIGURE 1 is a perspective view of a ring master havin g the shape to be reproduced;

FIGURE 2 is a cross-sectional view of the master shown in FIGURE '1 surrounded by a material for forming the wax pattern;

FIGURE 3 is a cross-sectional view, in part similar to FIGURE 2, showing the cutting of the surrounding material to remove the master and to form the Wax pattern;

FIGURE 4 is a cross-sectional view of the wax pattern;

FIGURE 5 is a cross-sectional, schematic view show ing the apparatus for forming a metallic coating upon a series of patterns positioned upon a tree;

FIGURE 6 is an enlarged cross-sectional view taken along the line 6-6 in FIGURE 5 subsequent to the depositing of the metal coating upon the tree;

FIGURE 7 is a cross-sectional View illustrating the metal covered tree surrounded by an investment;

FIGURE 8 is an enlarged cross-sectional view, in part similar to FIGURE 6, showing the mold after the wax pattern has been removed;

FIGURE 9 is a cross-sectional view, in part similar to FIGURE 8, showing the mold filled with the poured metal; and

FIGURE 10 is an enlarged view of the encircled area in FIGURE 7.

Referring now in detail to the drawings, the reference numeral 21 in FIGURE 1 identifies a master ring having the desired configuration which will be reproduced. The master ring 21 maybe formed by any known hand-forming operation or the like, as is well known in the art. The master ring 21 is surrounded by a body of latex or the like 22 (FIGURE 2) which flows around and completely encompasses the periphery of the master 21. The latex body 22 is then slit along a parting line, indicated generally by the reference numeral 23 (FIGURE 3), so that the master 21 may be removed. The body 22 is then adhered together along the parting line 23 and a sprue (not shown) is formed so that a body of wax 24 may be poured into a cavity 25 which has been left by removal of the master ring 21. Thus, a wax ring 24 (FIGURE 4) will be formed that will conform almost identically to the shape and surface texture of the master ring 21.

A plurality of the 'wax rings 24 formed in the described manner are supported upon rod-like wax members 25 which may be separate pieces or may be formed by the wax which solidifies in the sprue formed in the body 22. The rod-like members 25 are adhered, as by melting, to a gate 26 that is also formed of wax. The adherence of .a plurality of the rings 24 and rods 25 to the gate 26 forms what is known in the art as a tree, as indicated generally by the reference numeral 27 (FIGURE 5).

A thin metallic shell or coating is formed upon the tree assembly 27 by a metallizing process and apparatus, as depicted schematically in FIGURE 5. The metallizing apparatus is comprised of a vacuum jar 28 closed at its lower end by an integral wall and at its upper end by a removable cover plate 29. The tree assembly 27 is supported within the vacuum jar 28, as for example, upon a supporting grid 31. A heated filament 32 is wound around .a bar 33 formed from the metal that is to coat the tree assembly 27. In a preferred embodiment of the invention, the bar 33 is formed from pure aluminum. The filament 32 is suspended from the cover plate 29 by means of a pair of lead-in terminals 34 and 35 that extend through and are supported by insulating grommets 36 and 37 formed in the cover plate 29.

When the tree assembly 27 is appropriately positioned within the vacuum jar 28 and the cover plate 29 is in place, the space within the vacuum jar 28 is evacuated to form a partial vacuum therein by means of a vacuum pump (not shown). An electrical current is then caused to flow through the filament 32 heating it and the aluminumbar 33"as"well as providing" a' source of electrons. As is well known, the heated molecules of the bar 33 then evaporate and deposit upon the tree assembly 27, forming a thin shell aluminum coating indicated generally by the reference numeral 51, upon the tree (FIG- URE 6). The process is continued until the desired thickness of aluminum is deposited.

After the metal coating 51 is deposited upon the tree assembly 27 to the desired thickness, the metallic coated tree assembly is removed from the coating apparatus and is surrounded by an investment, for example a block of plaster of paris, indicated generally by the reference numeral 52 (FIGURE 7). The gate 26 is positioned so that it opens through the top of the plaster of paris block 52. After the metallic coated tree assembly is surrounded by the investment -52, it is heated sufficiently for the wax ring 24, rods 25 and gate 26 to melt and flow through the open end of the investment 52, leaving a hollow metallic mold which is formed by the metal 51 that was deposited upon the tree assembly 27 (FIGURE After all of the wax has been removed leaving the metallic mold, the opening of the tree gate 26 through the top of the investment 52 provides a pouring funnel into which molten metal such as gold or some other precious metal may be poured. The molten metal flows down the gate left by the removal of the wax through the risers provided by the removal of the rods 25 into cavities 53 formed from the removal of the wax rings 24 (FIG- URE 9).

The precious metal which is poured into the cavities 53 may be poured into them under a vacuum molding process in which the interior of the mold is evacuated and further, the metal may be centrifuged into the cavities 53 by rotating the investment 52 about a vertical axis that is intersected by the gate 26 to insure complete filling of the mold formed by the metallic shell 51. After the metal fills the cavity 53, as indicated by the reference numeral 54, and is sufficiently cooled, the investment 52 is removed by immersing it in water. This leaves a tree assembly having an outer shell of aluminum 51 and an inner core of the precious metal 54. The tree is then immersed in a strong alkali or other caustic bath (not shown) that will dissolve the aluminum shell 51 leaving the precious metal core 54 intact.

The disclosed casting method results in the formation of an extremely good surface finish that requires only a minimum of subsequent polishing. It will be noted from the enlarged view (FIGURE that the investment 52 which forms around the metallic shell 51 leaves a pluthe reference numeral 55 'due to its texture; In a'normal casting by the lost wax process, these voids remain after the wax has been removed. When the precious metal is poured into the void left by the wax, the precious metal will fill the voids 55 resulting in a rough surface finish on the completed article. This rough surface finish does not result in the disclosed method, however.

In this casting process, the inner periphery of the shell 51 actually forms the mold from which the ring is formed. Since the inner surface of the shell 51 is substantially the same as the outer periphery of the wax ring 24 and master 21, a surface finish which approximates that of the master 21will result. 'The metallizing of the wax tree assembly 27 will result in an extremely smooth surface on the inner side of the shell 51, which surface conforms exactly to the outer periphery of the wax tree assembly 27.

Although the disclosed method has been described in conjunction with the making of an ornamental finger ring, it is to be understood that the method may be practiced in the use of casting any object with a precision finish and dimension. Various other changes and modifications may be made without departing from the scope of the invention, as defined by the appended claims.

What is claimed is:

1. The method of making a precision casting comprismg the steps of forming a destructible non-metallic pattern, depositing a metal upon the pattern by a metallrzing process consisting of placing the pattern in at least a partial vacuum and heating a core of the metal to be deposited upon the pattern within the vacuum by encircling it with a heated filament, destructibly removing the non-metallic pattern from within the deposited metal to form a hollow metallic mold, surrounding the metallic mold by an investment, pouring a molten material into the metallic mold to form a casting and removing the metallic mold through destruction of the mold after the poured material has solidified.

References Cited UNITED STATES PATENTS 2,788,555 4/1957 Sukacev 164-34 3,064,112 11/1962 Hanzel 16498 X 3,139,658 7/1964 Brenner et al 164-46 3,237,253 3/1966 Webb 164-46 X 3,237,254 3/1966 Hanks et al. 164-50 I. SPENCER OVERHOLSER, Primary Examiner.

E. MAR, Assistant Examiner. 

